Is there an easy way to save work done in pprune ?
No. Unless of course you count simply posting your incomplete message then using the edit function to complete it.
I suggest tabbed browsing, or opening another instance of your browser, so that you can leave PPRuNe open while you do your photo stuff in another tab or instance.
Since you are composing in MS Word, you can save that as you go. I often do similar but with Notepad, because it has zero formatting so cuts & pastes nicely into any other application without dragging loads of unwanted formatting baggage with it.
SD